1. Field of the Invention
The present invention relates to a circuit for detecting the output power of a transmitter.
2. Description of the Related Art
A typical transmitter detection circuit for use in cellular telephones, etc., is now described with reference to FIG. 4.
A transmission signal is amplified by a power amplifier 51 to a predetermined output power, and is then transmitted from an antenna 52. A portion of the transmission signal from the power amplifier 51 is rectified and voltage doubled by a detection circuit 53 before the resulting signal is output as an AGC (automatic gain control) voltage to the power amplifier 51. The amplification of the power amplifier 51 is controlled so that the output voltage of the power amplifier 51 is constant.
The detection circuit 53 includes a first diode 55 to which a forward voltage is supplied by a bias voltage source 54, and a second diode 56 whose anode is connected to the cathode of the first diode 55. The anode of the first diode 55 is grounded via a DC blocking capacitor 61, and the cathode of the second diode 56 is grounded via a smoothing capacitor 62. The cathode of the second diode 56 is also coupled with the ground via a first resistor 57 and a second resistor 58 which are connected in series. The node between the first resistor 57 and the second resistor 58 is connected to an output port 63.
The operation of the thus constructed detection circuit 53 is now described.
A portion of the transmission signal from the power amplifier 51 is input to the node between the cathode of the first diode 55 and the anode of the second diode 56 via a charging capacitor 59 and a series resistor 60. The transmission signal is rectified and voltage doubled, and a voltage that is proportional to the output power is output from the node between the first resistor 57 and the second resistor 58, and is applied as an AGC voltage to the power amplifier 51 from the output port 63.
In such a structure, the detection circuit 53 has a narrow dynamic range, leading to a problem in that the dynamic range for the output power of the transmission signal is also narrow.
Accordingly, it is an object of the present invention to provide a detection circuit with a wide dynamic range.
To this end, according to the present invention, a transmitter detection circuit includes a first rectifying circuit for rectifying a transmission signal output from a power amplifier, and a logarithmic amplifier. The logarithmic amplifier includes a voltage-to-current converting circuit for outputting an electric current that is proportional to the output voltage of the first rectifying circuit, and a two-terminal p-n junction electronic device to which the electric current flows to produce a voltage thereacross that is proportional to the logarithm of the electric current.
The voltage-to-current converting circuit may include first and second transistors which perform differential operation, a constant-current source connected to one of the collector and the emitter of the first and second transistors, and a first current-mirror circuit connected to the other. The output voltage of the first rectifying circuit may be input to the base of the first transistor or the second transistor. The input end of the first current-mirror circuit may be connected to the first transistor, and the output end of the first current-mirror circuit may be connected to the second transistor, thereby causing an electric current that is equal to a difference between an electric current which flows to the second transistor and an electric current which flows to the transistor at the output end side of the current-mirror circuit to flow to the p-n junction electronic device.
The detection circuit preferably includes a second rectifying circuit having the same structure as that of the first rectifying circuit. The same bias voltage may be applied to rectifiers of the first and second rectifying circuits. The output voltage of one of the first and second rectifying circuits may be input to the base of the first transistor, and the output voltage of the other rectifying circuit may be input to the base of the second transistor.
The p-n junction electronic device may comprise a third transistor of which the base and the collector are coupled with each other.
The p-n junction electronic device may comprise a diode.
The voltage-to-current converting circuit and the p-n junction electronic device may be incorporated into a single integrated circuit.
The node between the first transistor or the second transistor and the first current-mirror circuit may be coupled with the ground or a power supply voltage via a first resistor.